Generally speaking, in human beings a normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood.

One common definition of thrombocytopenia is a platelet count below 50,000 per microliter.

Often, low platelet levels do not lead to clinical problems; rather, they are picked up on a routine full blood count (or CBC, complete blood count). Occasionally, there may be bruising, particularly purpura in the forearms, nosebleeds and/or bleeding gums.

It is vital that a full medical history is elicited, to ensure the low platelet count is not due to a secondary process. It is also important to ensure that the other blood cell types red blood cells, and white blood cells, are not also suppressed. Painless, round and pinpoints (1 to 3 mm in diameter), petechiae usually appear and fade, and sometimes group to form ecchymoses. Another type of blood leakage (and larger than petechiae), ecchymoses are purple, blue or yellow-green bruises that vary in size and shape. They can occur anywhere on the body from a traumatic injury.A person with thrombocytopenia may also complain of malaise, fatigue and general weakness (with or without accompanying blood loss). In acquired thrombocytopenia, the patient's history may include the use of one or several offending drugs.

Inspection typically reveals evidence of bleeding (petechiae or ecchymoses), along with slow, continuous bleeding from any injuries or wounds. Adults may have large, blood-filled bullae in the mouth. If the person's platelet count is between 30,000 and 50,000/mm3, bruising with minor trauma may be expected; if it is between 15,000 and 30,000/mm3, spontaneous bruising will be seen (mostly on the arms and legs).